Brominated polystyrene having improved thermal stability and color and process for the preparation thereof

ABSTRACT

A brominated polystyrene product made by the process comprising pretreating a solution of a polystyrene reactant comprising from about five to about 20 percent by weight of the polystyrene reactant, in a halogenated hydrocarbon solvent with at least about 0.1 percent by weight of an additive to suppress halogenation of the polystyrene backbone, prior to brominating the polystyrene in the presence of a catalyst; subsequently adding a metal halide bromination catalyst to the solution, capable of effecting bromination of the ring without causing crosslinking of the polystyrene; adding to the solution from about 1 to about 3.4 moles of a brominating agent, per mole of polystyrene repeating units; reacting the polystyrene with the brominating agent at a temperature of from about −20° C. to about 50° C., and isolating the brominated polystyrene. A brominated polystyrene is provided having a backbone halogen content of less than about 750 ppm and a ΔE color of from about 5 go about 19.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a Continuation of U.S. Ser. No. 09/076,944,filed May 13, 1998, presently pending, which is a Continuation-in-Partof U.S. Ser. No. 08/796,277, filed Feb. 7, 1997, U.S. Pat. No.5,726,252, which is a Division of U.S. Ser. No. 08/661,350, filed Jun.14, 1996, U.S. Pat. No. 5,637,650.

TECHNICAL FIELD

[0002] Brominated polystyrene is used as an additive to thermoplasticsto impart flame retardant properties. In addition to thermal stability,it is necessary and desirable for these additives to impart essentiallyno color to the thermoplastic. The evolution of engineeringthermoplastics has resulted in specialty polymers with much higher heatresistance and, as a result, a need to process these new materials atever increasing temperatures. Because of higher and higher processingtemperatures, the flame retardant additives used in these engineeringthermoplastics must have a higher order of thermal stability and bettercolor than that required in the past. Accordingly, this inventiongenerally relates to a brominated polystyrene having improved color andthermal stability. More particularly, the invention relates to a processfor the bromination of polystyrene which overcomes the limitations ofcurrent technology by use of an additive to suppress backbonehalogenation.

BACKGROUND OF THE INVENTION

[0003] Reports of the use of brominated polystyrene as a flame retardantadditive in thermoplastics extend back more than twenty-five years. In1980, Ferro Corporation, the Assignee of record herein, introducedbrominated polystyrene as a commercial flame retardant additive underthe trade name PyroChek® 68PB. The process for producing PyroChek® 68PBis described in U.S. Pat. No. 4,352,909. This product has become aleading flame retardant additive for use in reinforced engineeringthermoplastics. More recently, Great Lakes Chemical has introduced asecond brominated polystyrene product, PDBS-80, to the marketplace. Thisproduct also finds its primary application in engineeringthermoplastics.

[0004] Thus, there are currently two different synthetic routesavailable for the commercial production of brominated polystyrenes. Eachprocess has certain advantages and disadvantages which should be notedin order to fully understand the significance of the present invention.

[0005] The process used to produce PDBS-80, the commercial productoffered by Great Lakes Chemical, is described in U.S. Pat. No.5,369,202. It involves four chemical steps starting from styrenemonomer. The first step involves the addition of HBr across the doublebond of the styrene in order to protect it. In the second step, thisintermediate is brominated on the ring using conventional technology.Usually an average of two bromines are introduced. The secondintermediate is then reacted with strong inorganic base. This eliminateshydrogen bromine from the bromoethyl group of the second intermediate,reforming the double bond to produce brominated styrene monomer. Afterpurification, this monomer is polymerized to form the brominatedpolystyrene product. The entire process may be represented as follows:

[0006] This process has one significant advantage. It produces abrominated polystyrene which is essentially free of backbone halogen.This results in a product with very good thermal stability, good color,and good color stability. However, the process has two seriouslimitations which are major disadvantages when compared to the alternateprocess.

[0007] a. The process involves four distinct chemical reactions as wellas several other unit operations. It is a complex process requiring acomplex manufacturing facility with a high capital cost and themultiplicity of steps results in a long process. This process isinherently expensive.

[0008] b. Brominated styrene monomers are very reactive and difficult tohandle. Ideally, a brominated aromatic flame retardant additive shouldhave a high bromine content in order to have maximum efficiency andminimum cost. Thus, it would be preferable to produce and polymerizetribromostyrene monomer. However, this monomer is a highly reactivesolid with low volatility. It is difficult to handle and polymerize andany residual monomer in the polymer would be difficult to remove.Consequently, this process tends to be limited to dibromostyrene as themaximum degree of bromination practical by this process. This limits thebromine content of the commercial brominated polystyrene (PDBS-80) toabout 60%. Consequently, when used as a flame retardant additive, arelatively high use level is required to achieve flame retardance. Thismakes the product expensive to use. But of even greater concern to theuser is the fact that high use levels cause deterioration of theimportant physical properties of the host resin. This result isfrequently unacceptable to the user.

[0009] The process used by Ferro Corporation to produce its brominatedpolystyrene flame retardant additive, PyroChek® 68PB is described in theaforementioned U.S. Pat. No. 4,352,909. This process has many advantagesover the process which involves the production and polymerization ofbrominated styrene monomer. Some of these include:

[0010] a. The process involves only a single chemical reaction, thebromination of commercially available polystyrene dissolved in acommercially available solvent using a commercially availablebrominating agent, bromine chloride. The process can be carried out in asimpler plant with a much lower capital cost. This process is inherentlyless expensive than the production of brominated polystyrene by thepreparation and polymerization of brominated styrene monomer.

[0011] b. Because the process never involves the formation and handlingof brominated styrene monomer, it does not have the limitations of theother process. It is possible to achieve tribromination and approachbromine contents of 70%. Since the brominating agent is less expensivethan the polystyrene raw material, this actually reduces the cost of theproduct. Further, higher bromine contents result in lower use levels toachieve flame retardance. This reduces costs. But of even greaterimportance, reduced use levels result in better retention of physicalproperties of the host resin.

[0012] c. The process allows the use a wide variety of polystyrenes andthis, in turn, allows for the production of a variety of brominatedpolystyrenes. Further, general purpose, crystal polystyrene is producedin very large volumes in every part of the developed and developingworld. This makes it readily available and inexpensive.

[0013] Notwithstanding the many advantages this process has over theprocess for making brominated polystyrene from monomer, a disadvantageexists which is beginning to limit the value and versatility of thisproduct. In particular, while the process puts most of the bromine onthe aromatic ring of the polystyrene, it also puts a small butsignificant amount of bromine and chlorine on the backbone. Typically,the amount of halogen, reported as HBr, on the backbone is 5000-6000ppm, as measured by a test procedure described in detail hereinbelow.This backbone halogen is the direct cause of the limited thermalstability of brominated polystyrenes produced in this manner and is thedirect cause of both its problems regarding initial color and colorstability during thermal processing. Under the conditions of thermalprocessing, the backbone halogen of the current brominated polystyrenesproduced in this manner may be released causing corrosion of processingequipment and degradation of the host resin. The formation ofunsaturation in the backbone of the brominated polystyrene also leads toa loss of good color during processing. Since the technology trend inengineering thermoplastics is to higher and higher processingtemperatures, the current brominated polystyrenes produced in thismanner are becoming less acceptable in newer applications.

[0014] When brominated polystyrene is employed as a flame retardantadditive in thermoplastics, its color is a property of primaryimportance to the manufacturer of the thermoplastic materials. Thethermoplastic manufacturer desires to produce the thermoplastic articlesin a wide range of colors. The more highly colored an additive, the moredifficult it becomes to match (produce) a broad range of colors. Themore lightly colored the additive, the easier it becomes to produce awide range of colors. Therefore, in view of the needs of themanufacturer of thermoplastic parts, and in view of the inadequacy ofprior art processes to produce a highly brominated polystyrene havingthe desired light color characteristics, a need exists for a highlybrominated polystyrene with an improved light appearance as manufacturedso that the end user can formulate a wide range of colors and therebybetter meet the needs and demands of the marketplace.

SUMMARY OF THE INVENTION

[0015] It is therefore, an object of the present invention to provide aprocess for highly brominating polystyrenes which allows the directbromination of polystyrene to produce a product with excellent thermalstability, excellent color, good color stability, and a minimum ofbackbone halogen.

[0016] It is another object of the present invention to provide aprocess which can be carried out in the existing facilities for thebromination of polystyrene without modification, without any additionalcapital investment, and with an absolute minimum of increase in rawmaterial cost.

[0017] It is yet another object of the present invention to provide aprocess which utilizes an additive for the suppression of backbonehalogenation, thereby allowing the operator to obtain highly brominatedpolystyrenes having improved thermal stability and color.

[0018] It is another object of the present invention to provide a highlybrominated polystyrene having improved color and thermal stability andwith less backbone halogen.

[0019] At least one or more of the foregoing objectives, together withthe advantages thereof over existing prior art forms, which shall becomeapparent from the specification which follows, are accomplished by theinvention as hereinafter described and claimed.

[0020] In general, the present invention provides a brominatedpolystyrene product made by the process comprising pretreating asolution of a polystyrene reactant comprising from about five to about20 percent by weight of the polystyrene reactant, in a halogenatedhydrocarbon solvent with at least about 0.1 percent by weight of anadditive to suppress halogenation of the polystyrene backbone, prior tobrominating the polystyrene in the presence of a catalyst; subsequentlyadding a metal halide bromination catalyst to the solution, capable ofeffecting bromination of the ring without causing crosslinking of thepolystyrene; adding to the solution from about 1 to about 3.4 moles of abrominating agent, per mole of polystyrene repeating units; reacting thepolystyrene with the brominating agent at a temperature of from about−20° C. to about 50° C., and isolating the brominated polystyrene.

[0021] The present invention also provides a brominated polystyrenehaving a backbone halogen content of less than about 750 ppm and, a ΔEcolor of from about 5 to about 19.

PREFERRED EMBODIMENT FOR CARRYING OUT THE INVENTION

[0022] Brominated polystyrene, marketed primarily as PyroChek® 68PB byFerro Corporation, the Assignee of record herein, has been a leadingflame retardant additive for engineering thermoplastics for many years.As currently produced, it can contain anywhere between 3000 to 6000 ppmand typically contains 5000 to 6000 ppm of backbone halogen, measured asHBr. This latter factor is the primary source of the poor thermalstability of the additive which, in turn, is a problem at higherprocessing temperatures. Furthermore, it has a ΔE color, measured as asolution in chlorobenzene and compared to the color of purechlorobenzene. Total Color Difference (ΔE) is determined using theHunter L, a, b scales, for product solutions in chlorobenzene. Productsof the present invention have a ΔE color of up to about 19, andpreferably about 18, and a considerably lower backbone halogen content.

[0023] One preferred embodiment of the process of the present inventionmay be represented by the following equation, involving two steps:

[0024] In Equation I, the term BHSA refers to backbone halogenationsuppressing additive which will be explained later. As Equation Iindicates, the reaction in this embodiment of the invention is generallyconducted in a solvent, preferably a chlorinated hydrocarbon solvent. Instep (1) the BHSA, solvent and polystyrene are combined. Preferredsolvents include halogenated hydrocarbons such as carbon tetrachloride,chloroform, methylene chloride, 1,2-dichloroethane, 1,2-dibromoethane,1,1,2-trichloroethane, 1,1,2,2-tetrachloroethane and the like. Thepreferred solvent is EDC (1,2-dichloroethane). Mixtures of solvents canalso be employed.

[0025] The polystyrene reactant that is employed may be either anoligomer or a polymer. Accordingly, the initial molecular weight of thepolystyrene is from about 500 {overscore (M)}w to about 1,500,000{overscore (M)}w and preferably from about 500 {overscore (M)}w to about500,000 {overscore (M)}w. The process is also effective for thebromination of substituted polystyrene, the substitution being nuclear.Obviously, nuclear substituents will affect the position(s) at which thebromination occurs and the amount of additional bromination that takesplace. Examples of the substituted polystyrenes that may be brominatedin accordance with the process of the invention include halogenated andalkylated polymers such as poly-(bromostyrene), poly-(chlorostyrene),poly-(dichlorostyrene), poly-(dibromostyrene),poly(chloro-bromo-styrene), poly-(4-methyl styrene) and poly-(mono-loweralkyl styrene). Halogen substituents include chlorine and bromine andalkyl substituents include lower alkyl group having from one to aboutfour carbon atoms. Accordingly, the term polystyrene reactant, or justpolystyrene, as used throughout the specification and claims, shallrefer to the foregoing homopolystyrene and oligomers as well assubstituted polystyrenes within the scope of this invention.

[0026] The reaction is carried out to introduce up to three bromineatoms on each aromatic nucleus. Hydrogen chloride or hydrogen bromide isproduced as a byproduct of the reaction, depending upon whether brominechloride or bromine is used.

[0027] While the invention can be employed, as indicated in Equation Iabove, for the production of what is essentially tribrominatedpolystyrene, the process of the invention is of general utility for theproduction of brominated polystyrene products having any desired degreeof bromination up to three.

[0028] Prior art bromination techniques, applied to styrene polymers oroligomers, are currently less effective than the present process inproducing a suitably light colored material. Products can be produced bythe preferred process of the invention at any desired level ofbromination with very good color characteristics, i.e., very light incolor, so that the highly brominated products are desirable flameretardant additives for the plastics industry. Products having a lowerdegree of bromination than essentially tribromination are also useful asflame retardant additives.

[0029] U.S. Pat. No. 4,352,909 provides an adequate description of thecurrent commercial process for brominating polystyrene and accordingly,the subject matter of this patent is incorporated herein by reference.Briefly summarized, that process involves dissolving the polystyrene tobe brominated in a suitable solvent (usually ethylene dichloride) whichis reasonably dry (less than 150 ppm of H₂O). A catalyst is added,usually antimony trichloride, and then the brominating agent, usuallybromine chloride, is added gradually while controlling the temperature,usually between 20 and 40° C. When the reaction is complete, the excessbrominating agent is decomposed by the addition of a reducing agent,usually aqueous sodium bisulfite. The aqueous phase is separated and theorganic phase is washed with water several times to remove residualacid. While the product can be isolated using any one of severalmethods, the normal procedure involves gradual addition of the solutionof product in ethylene dichloride to very hot water. This causes thesolvent to flash distill off, leaving the product as a slurry in water.The product is then dewatered by centrifugation and finally dried byflash and/or rotary vacuum drying.

[0030] The present invention requires only one minor but extremelyimportant change in the above procedure. After the polystyrene isdissolved in the solvent and before addition of the bromination catalystand brominating agent, an additive to suppress halogenation of thepolystyrene backbone, or backbone halogenation suppressing additive(BHSA) is dissolved in the solution and the mixture is stirred for up toan hour. After this step (1) is completed, the remainder of the processis carried out in the usual manner, involving the addition ofbrominating agent and catalyst, step (2).

[0031] While the effect on backbone halogenation suppression occursmerely upon addition of the BHSA, optimum benefit occurs after someperiod of agitation once the BHSA has been added and prior to additionof the bromination catalyst and the brominating agent. In other words,zero addition time works, which is defined as adding the BHSA first andimmediately proceeding to add the catalyst and brominating agent.However, in most instances it is preferable to pretreat the polystyrenein solution in the presence of the BHSA. Pretreatment can last for asfew as 15 minutes, up to several hours. As a practical matter, whileextended periods of pretreatment are not deleterious, after one toseveral hours it is considered desirable to proceed with the remainingsteps of the process rather than tie up the reaction vessel andequipment unnecessarily. During the step of pretreating, the solutionwith BHSA can be agitated in a conventional manner. The addition of BHSAand pretreating step may be conducted at temperatures ranging betweenabout −20° and 50° C. and at standard pressure.

[0032] With respect to catalyst selection, bromination is viaFriedel-Crafts catalysis and thus, metal halides are employed. Thecatalyst must satisfy two criterion, first, it must be capable ofcarrying out the bromination and second, it must do so without causingconcurrent crosslinking of the polystyrene via Freidel-Craftsalkylation. Recognizing that the system contains aliphatic halogenseither from the polystyrene backbone or the solvent, or both, thecatalyst selected should not effect reaction between the two causingcrosslinking. Those skilled in the art can readily determine suitablecatalysts as it is well known first, which of the known metal halidescan catalyze the reaction and second, those that will not effect theundesired crosslinking. The literature is replete with such references,a suitable list being available in Journal of the American ChemicalSociety, Vol. 94, pp 7448-7461, George A. Olah, Shiro Kobayashi andMasashi Tashiro (1972).

[0033] Returning to the process of the present invention, while thisprocess change, involving the use of a BHSA, can be viewed as minimaland having little economic impact, the effect on the quality of theproduct is quite profound. The amount of backbone halogenation isreduced as much as 95% and the color is improved by as much as 50%.Further, the color stability during thermal processing is greatlyimproved. The product has all of its original advantages without thedisadvantage of high backbone halogen which results in poor color and/orthermal stability.

[0034] Reduction of backbone halogen content can be achieved with theaddition of as little as 0.1% by weight (based on polystyrene charge) ofthe backbone halogenation suppressing additive (BHSA). Relatively largeamounts of BHSA, on the order of 15 to 20%, can be added withoutsignificant negative consequence. However, addition beyond the optimumlevel does not result in further reduction of backbone halogen content.The best balance of performance and cost is typically achieved when theBHSA level is between one and five percent, by weight of the polystyrenecharge.

[0035] U.S. Pat. No. 5,723,549 describes several normal reactionparameters which, if properly controlled, can enhance the color ofbrominated polystyrene. These include reaction time and temperature,choice of catalyst and brominating agent, and the method of isolation.The effect of these variables is also observed when BHSA is employed toreduce backbone halogenation. However, several things should be kept inmind. First, the variables described in U.S. Pat. No. 5,723,549 do notprovide any positive control over backbone halogenation. Second, theimpact of these variables on color is diminished when BHSA is employed.For example, conducting the bromination at lower temperatures willimprove product color when BHSA is employed. However, the improvement isnot as great as without the use of the BHSA. When BHSA is employedalmost all the color improvement is due to suppression of backbonehalogenation. It should be noted that the use of a BHSA not only greatlyreduces backbone halogenation but its use also results in a much moreforgiving process. This has great practical value in the commercialoperation of a production facility.

[0036] Unexpectedly, by the addition of the backbone halogenationsuppressing additive (BHSA) to the solution of the polystyrene reactantin the chlorinated hydrocarbon solvent, the backbone halogen, measuredas HBr, is reduced by as much as an order of magnitude over conventionalprocesses for preparing brominated polystyrenes. As an example, backbonehalogen can be lowered from about 3000 to 6000 ppm to about 750 ppm, andpreferably at least about 250 ppm, for a reduction of as much as about80 to 95%. Moreover, color is improved when the additive is employedsuch that a ΔE color of less than from about 6.7 to about 7.8 and as lowas 5 is obtainable in comparison to a conventional ΔE color of between13 and 16, a reduction of between about 30 to 60%. Nevertheless,products having a conventional ΔE color of between 13 and 16 have notpossessed low backbone halogenation and thus, products according to thepresent invention having a backbone halogen of less than about 750 ppmcan have a ΔE color of from about 5 up to about 19, the upper valuestill being acceptable for brominated polystyrene products. Morepreferably, the upper value is about 18. Thermal stability of thebrominated polystyrene prepared according to the process of the presentinvention is also improved due to the lowered backbone halogen content.

[0037] Thermal stability of brominated polystyrene is improved forpolystyrene products that are essentially or predominantly ringhalogenated versus ring and backbone halogenated. Accordingly, whenbrominating polystyrene, the ideal result is to place 100% of thehalogen on the aromatic ring of the polystyrene and to have no halogenon the backbone of the polymer. By way of explanation, bonds betweenbenzylic (backbone) carbon atoms and halogen atoms are lessthermodynamically stable than bonds between aromatic ring carbons andhalogen atoms. For example, the bond dissociation energy of aC(benzylic)—Br bond is 51 kcal/mole while that of a C(aromatic)—Br bondis 71 kcal/mole. This means that a C(benzylic) Br bond will breakdown ata lower temperature than the C(aromatic)—Br bond. When this occurs, thevery corrosive hydrogen bromide is released and a double bond is formed.As the number of double bonds in the backbone increases, the colorquality of the brominated polystyrene will diminish. Hence backbonehalogenation is to be avoided.

[0038] There is a graphic demonstration of the stability of ring brominevs backbone bromine. It is possible, on a laboratory scale, to producepoly(tribromostyrene) from tribromostyrene monomer. Brominatedpolystyrene made in this way contains no backbone halogen. It contains70.3% bromine, all of which is on the ring. On the other hand,brominated polystyrene made commercially via the process of U.S. Pat.No. 4,352,909 contains backbone halogen. There is a thermal stabilitytest procedure detailed hereinbelow which involves heating thebrominated polystyrene for 15 minutes at 300° C. and measuring the totalamount of hydrogen bromide liberated during the test period. When thisis done on the commercial brominated polystyrene, 3000-6000 ppm of HBrare liberated. When the same test is carried out onpoly(tribromostyrene) made from monomer no HBr was detected. This showsthat aromatic bromines are 100% stable at 300° C. while backbone halogenis sufficiently unstable to be eliminated well below 300° C.

[0039] The benefits of the process of the present invention are a resultof the use of titanium tetrachloride as the additive. Other additivesthat could be employed include tin tetrachloride and boron trichloride,which are exemplified hereinbelow.

GENERAL EXPERIMENTAL

[0040] All of the data generated and reported herein to define practiceof this invention has been conducted on a laboratory scale. In order tomake a fair comparison, the properties obtained using the process of thepresent invention have been compared to those of commercial brominatedpolystyrene when it is produced on a laboratory scale. Under theseconditions, PyroChek® 68PB has a color ΔE of 13 to 16 and a backbonehalogen of 3000 to 6000 ppm, with 5000 to 6000 ppm being typical,measured as HBr, evolved while heating a measured sample for 15 minutesat 300° C.

[0041] The invention will now be further described in detail bydescriptions of specific demonstrations thereof. In the followingexamples and throughout this application, all parts and percentages areby weight and all temperatures are expressed in degrees Celsius, unlessexpressly stated to be otherwise. All reactions were conducted atstandard pressure.

THERMAL STABILITY TEST

[0042] Engineering thermoplastics (ETP's) have enjoyed tremendous growthduring the past twenty-five years. One reason for this is that thisclass of materials, particularly the reinforced grades, have excellentheat resistance which makes them particularly useful for continuous useat elevated temperatures. In recent years, the development of newengineering thermoplastics has focused on materials with everyincreasing heat resistance. This has been accomplished by creating newpolymers with higher glass transition temperatures (Tg) or highermelting points (Tm). However, the development of these new materials hasnot been without complications. As the Tg or Tm increased substantially,the temperatures required to process these materials also underwentsubstantial increases. Fifteen years ago, ETP's were rarely processed atmelt temperatures approaching 300° C. Today, that processing temperatureis quite common and new materials may now go as high as 350° C. whenbeing processed.

[0043] In order to be useful, all the additives used to formulate thenewer ETP's must have sufficient thermal stability to survive the higherprocessing temperatures. This is especially true of the halogencontaining additives used to impart flame retardance to ETP's. If theseadditives have inadequate thermal stability, they will degrade whenprocessed at high temperatures and liberate hydrogen chloride and/orhydrogen bromide which are very corrosive. If these materials arereleased during processing they may degrade the ETP or, at the veryleast, cause corrosion of the equipment used to process thethermoplastic. If this latter process occurred, it could cause seriousdamage to the equipment. This, in turn, would result in major expensesto repair the equipment and a loss of production time on the equipment.Obviously, a user of halogenated flame retardant additives would begreatly concerned about the thermal stability of the additives that theyuse.

[0044] As a supplier of halogenated flame retardant additives, FerroCorporation felt that is was very important to develop a simplescreening test which would allow for the evaluation of the relativethermal stability of various halogenated flame retardants. Conceptually,the procedure is very simple. A carefully measured sample (2.00±0.01 g)of additive is exposed to a temperature of 300° C. for fifteen minutes.All of the acidic gases (HCl, HBr) generated during the period arecollected in a standard solution of NaOH. This solution is acidified toa pH <7 and then is titrated with standardized silver nitrate using apotentiometric tetrameter. This measures the parts per million of HCland HBr that were released during the heating period. In the interest ofsimplicity, the ppm of HCl are converted into ppm HBr, this is added tothe amount of HBr already measured, and the resultant number is reportedas ppm of HBr equivalent. The larger the amount of HBr equivalentreported, the less thermally stable is the given additive. An additivereleasing 0 ppm of HBr equivalent would have the best thermal stability.A detailed description of the test procedure follows.

THERMAL STABILITY TEST PROCEDURE

[0045] The apparatus is assembled in a fume hood. A 2.00±0.01 g sampleis weighed in a 20×150 mm tared test tube. Three 250 mL sidearm filterflasks are filled with 150-170 mL of 0.1N NaOH (enough to completelycover the frit) containing phenolphthalein (2% w/v solution in 3A EtOH),and are connected with Viton® tubing. This allows the acidic gasesgenerated by a sample in the test tube to be passed through the aqueousNaOH, thus trapping the HBr and/or HCl (HX). The test tube containingthe sample is fitted with a no. 2 neoprene stopper with a {fraction(1/16)}″ inlet and a 7 mm outlet for Teflon® tubing. The sample ispurged with N₂g) (flow rate=0.5 SCFH) for five minutes, then placed inthe salt bath deep enough to surround the entire sample for 15 minutes.The sample is withdrawn from the bath and purged for another fiveminutes. The test tube containing the pyrolyzed sample is removed andreplaced with a clean empty test tube. This test tube with the N₂g purgeis submerged in the salt bath for five minutes to flush out any residualHX.

[0046] After the test tube is rinsed, the gas dispersion tubes arecarefully removed and rinsed with deionized (di) H₂O, keeping N₂(g) flowthrough the test tube during the rinse. Begin with the last collectionflask and work back to the first. After all dispersion tubes are out,the empty test tube is removed. The Viton® tubing connecting each of theflasks is also rinsed with di H₂O. The contents of the flasks arecombined and quantitatively transferred to bottles, rinsing with di H₂O,until the operator is ready to do titrations (described below). Thesolutions can be stored in these bottles with caps if the solution iskept alkaline. Two or three test tubes containing no sample are run asblanks before the first sample each day of testing in order to verifythat there is no residual HX in the system.

[0047] Once the samples have been pyrolysed and the HX gases collected,the bottled solutions are titrated in the analytical lab using a Metrohm670 titroprocessor with an Ag combination electrode. Each samplesolution is acidified with a 1:2 solution of HNO_(3;) DI H₂O, to a pH<7,and then filtrated with standardized AgNO₃ to a potentiometricequivalence point. The parameters for the filtration are those which arerecommended in the manual for the titroprocessor. Variations of thoseparameters are left to the discretion of the operator. The results arereported in duplicate as ppm HBr HCl, and ppm HBr Equivalents.

[0048] Calculations:

[0049] ppm HBr=(Ep1 mL *^(N) titrant * molecular wt. HBr *1,000,000)/(wt. of Sample * 1000)11

[0050] ppm HCl=[(Ep2 mL-Ep1 mL) *^(N) titrant * molecular wt. HCl *1,000,000]/(wt. Sample * 1000)

[0051] ppm HBr Eq={[Ep2 mL-Ep1 mL) *^(N) titrant * molecular wt. ofHBr * 1,000,000]/(wt. Sample * 1000)}+ppm HBr

[0052] where Ep=end point volume in mL

[0053] and ^(N) titrant=Normality of AgNO₃

EXPERIMENTAL PROCEDURE

[0054] PyroChek® PB68 was produced in the laboratory by dissolvinggeneral purpose polystyrene in ethylene declared (EDC). Antimonytrichloride (5% by weight based on polystyrene charged) was added as acatalyst. Then the brominating agent, bromine chloride, containing 10%EDC, was added gradually while maintaining the reaction temperature at20° C. The total reaction time was five hours to produce the productdescribed hereinabove.

[0055] In the present invention, the process is identical to the generalprocess above, with one important exception and that is, prior to theaddition of the catalyst and the initiation of bromination, the backbonehalogenation suppression additive was added to the solution of EDC thesolution was agitated for 30 minutes. Thereafter, the general processwas followed.

[0056] In the first series of experiments the level of BHSA employed wasvaried to determine the effect on product quality. All experiments werecarried out at 20° C. with a 5 hour reaction time. The solvent wasethylene dichloride. The bromination catalyst was SbCl₃ used at a levelof 0.023 moles per mole of polystyrene. The polystyrene solutioncontaining the BHSA was agitated for 30 minutes before adding thebromination catalyst and initiating BrCl addition. Example No. 1 was aControl, made according to U.S. Pat. No. 4,352,909, without any BHSA.Color was determined as Total Color Difference (ΔE), using the Hunter L,a, b scales, for product solutions in chlorobenzene, 10 percent byweight concentration versus chlorobenzene, according to the formula:

ΔE={square root}{square root over ((ΔL)²+(Δa _(L))²+(Δb _(L))²)}

[0057] Results are reported in Table 1. TABLE I EFFECT OF BHSA LEVEL ONPRODUCT QUALITY Ex. BHSA Level⁽¹⁾ Color HBr Equivalent No. (Moles/MolePS) (ΔE)⁽²⁾ (ppm)⁽³⁾ 1 0 14.5  6000  2 0.0055 9.6 912 3 0.027 6.8 531 40.055 7.3 673 5 0.082 8.5 568

[0058] It will be noted that backbone halogenation was decreased as theamount of BHSA was increased and, that the content of backbone halogenwas decreased by an order of magnitude over the Control. Color was alsoimproved over the Control by the use of a BHSA.

[0059] In the next series of experiments, the level of BHSA employed wasconstant and the effect of agitation time was varied to determine theeffect on product quality. Example No. 1 was the Control from Table I,without any BHSA. All experiments were carried out at 20° C. with a 5hour reaction time. The bromination catalyst was SbCl₃ used at a levelof 0.023 moles/mole of polystyrene. The solvent was ethylene dichloride.TABLE II EFFECT OF AGITATION TIME Agitation HBr Ex. BHSA Level⁽¹⁾Time⁽²⁾ Color⁽³⁾ Equivalent⁽⁴⁾ No. (Moles/Mole PS) (Minutes) (ΔE) (ppm)1 None 0 14.5 6000 6 0.082 0 15.2 1002 7 0.082 30   8.5  658

[0060] The use of a BHSA without prior agitation greatly reduced thelevel of backbone halogenation. However, the best results were obtainedwhere the solution of polystyrene and BHSA was stirred together for atleast 30 minutes.

[0061] In the next series of experiments, the brominating agent brominewas employed. Example No. 8 was another Control from Table I, withoutany BHSA. All experiments were carried out at 20° C. with a 5 hourreaction time. The bromination catalyst was SbCl₃ used at a level of0.023 moles/mole of polystyrene. In the experiment with the BHSA, thesolution of polystyrene and BHSA was agitated for 30 minutes beforeinitiation of bromination. The solvent was ethylene dichloride. TABLEIII WITH BROMINE AS THE BROMINATING AGENT Ex. BHSA Level Color HBrEquivalent No. (Moles/Mole PS)¹ (ΔE)² (ppm)³ 8 0 21.2 5939 9 0.082 10.5 750

[0062] As evident from the foregoing results, the use of a BHSA iseffective when bromine is used as the brominating agent.

[0063] In the next series of experiments, the effect of differentsolvents was considered, using the same amount of BHSA. Example No. 1was the Control from Table I, without any BHSA. All experiments werecarried out at 20° C. with a 5 hour reaction time. The brominationcatalyst was SbCl₃ used at a level of 0.023 moles/mole of polystyrene.The polystyrene solution containing the BHSA was agitated for thirtyminutes before adding the bromination catalyst and initiating BrCladdition. TABLE IV EFFECT OF SOLVENT HBr Ex. BHSA Level¹ ColorEquivalent No. (Moles/Mole PS) Solvent (ΔE)² (ppm)³ 10 0.082 ClCH₂CH₂Cl 8.6 602 11 0.082 ClCH₂CH₂Cl  8.5 568 12 0.082 CH₂Cl₂ 11.7 624 13 0.082CH₂Cl₂ 11.5 658  1 None ClCH₂CH₂Cl 14.5 6000 

[0064] In the next series of experiments, two other materials wereemployed as the BHSA and compared against titanium tetrachloride and theControl, Example No. 1 from Table I, without any BHSA. All experimentswere carried out at 20° C. with a 5 hour reaction time. The brominationcatalyst in each experiment was SbCl₃ at a use level of 0.023 moles permole of polystyrene. The level of BHSA was 0.082 moles per mole ofpolystyrene. The solvent was ethylene dichloride. The polystyrenesolution containing the BHSA was stirred for 0.5 hours before adding thebromination catalyst and initiating the addition of BrCl. TABLE VREPRESENTATIVE ADDITIVES PROVIDING SUPPRESSION OF BACKBONE HALOGENATIONEx. Color HBr Equivalent No. BHSA (ΔE)¹ (PPM)²  1 None 14.5  6000  14TiCl₄ 8.5 568 15 SnCl₄ 9.7 262 16 BCl₃ 13.7  267

[0065] It will be noted that backbone halogenation was decreased andcolor was also improved over the Control by the use of all three BHSAmaterials. Again, the content of backbone halogenation was decreased byan order of magnitude over the Control when a BHSA was employed.

[0066] In view of the foregoing results, the brominated polystyrene ofthe present invention provides a bromine content of at least about 66percent by weight; backbone halogen, conventionally between 3000 to 6000ppm, is reduced by as much as 95% and preferably 80 to 95% to less thanabout 750 ppm and preferably 250 ppm; and, a ΔE color conventionallybetween 13 and 16, for products having the higher, conventionally knownbackbone halogen content, is reduced by as much as 60% and preferably 40to 60% to less than about 7 and as low as about 5. Given the lowbackbone halogen of less than about 750 ppm, ΔE color for the brominatedpolystyrenes of the present invention ranges from about 5 up to about 19and preferably about 18. Thermal stability of the brominated polystyreneis assured and is increased because the backbone halogenation is so muchlowered compared to conventional brominated polystyrenes.

[0067] Thus it should be evident that the process of the presentinvention is highly effective in preparing a brominated polystyrenehaving improved thermal stability and color.

[0068] Based upon the foregoing disclosure, it should now be apparentthat the use of the process described herein will achieve the objectivesset forth hereinabove. It is, therefore, to be understood that anyvariations evident fall within the scope of the claimed invention andthus, the selection of specific component elements can be determinedwithout departing from the spirit of the invention herein disclosed anddescribed. In particular, the brominating agent, catalysts and reactiontemperatures and times and other reaction conditions according to thepresent invention are not necessarily limited to those discussed herein.Nor, is practice of the present invention necessarily limited to the useof titanium tetrachloride, tin tetrachloride or boron tricolored as theadditive to suppress backbone halogenation during the bromination ofpolystyrenes. Thus, the scope of the invention shall include allmodifications and variations that may fall within the scope of theattached claims.

That which is claimed:
 1. A process for the production of brominatedpolystyrene by the bromination of polystyrene, the process comprising:contacting the polystyrene prior to its bromination with a solutioncontaining an additive that suppresses halogenation of the polystrenebackbone during the bromination.
 2. The process of claim 1 wherein thesolution solvent is a halogenated hydrocarbon.
 3. The process of claim 1wherein the solvent is selected from the group consisting of carbontetrachloride, chloroform, methylene chloride, 1,2-dichloroethane,1,1,2- trichloroethane, 1,1,2,2-tetrachloroethane, 1,2-dibromoethane andmixtures of two or more of the foregoing.
 4. The process of claim 1wherein the bromination is effected with bromine or bromine chloride. 5.The process of claim 1 wherein the bromination is effected with brominechloride and is catalyzed with antimony trichloride, antimony tribromideor mixtures thereof.
 6. The process of claim 1 wherein the additivecomprises titanium tetrachloride, tin tetrachloride or borontrichloride.
 7. The process of claim 6 wherein the solvent is selectedfrom the group consisting of carbon tetrachloride, chloroform, methylenechloride, 1,2-dichloroethane, 1,1,2- trichloroethane,1,1,2,2-tetrachloroethane, 1,2-dibromoethane and mixtures of two or moreof the foregoing.
 8. The process of claim 7 wherein the bromination iseffected with bromine or bromine chloride.
 9. The process of claim 1wherein the bromination is effected with bromine chloride.
 10. Theprocess of claim 7 wherein the bromination is effected with brominechloride.